[Greenbuilding] HDD and how they are calculated (daily avg. vs hourly avg)

[Reuben Deumling]

My understanding is that NOAA simply averages the high and low temperature
in a 24 hour period and subtracts that average from 65F. But if one were to
say use hourly data, the resulting HDD figure would be different. How much
different I don't know. I suspect there are efforts to calculate the error
(for a day or month) but haven't found it. Anyone?

http://knol.google.com/k/degree-days#

"The calculation method that I explained above is essentially the
*correct*one for calculating heating degree days: for each period over
which the
outside air temperature was constant, you multiply the degrees below the
base temperature by the number of days that the temperature was fixed for
(usually small fractions of days), and then you sum all the values together
to get the total heating degree days for the period in question.

The problem with that approach is that, in the real world, outside air
temperature doesn't remain constant - in fact it changes pretty much all
the time.  Mathematically speaking you'd need an *infinite* number of
temperature readings to calculate degree days properly.

Fortunately, "mathematically speaking" doesn't really matter too much in
this instance, and half-hourly or hourly temperature readings are plenty
good enough to calculate degree days accurately using the method described
above.

However, reliable half-hourly and hourly temperature readings are rarely
readily available, so there are a number of other approximation methods
that are used to calculate degree days from more commonly available
measurements of outside air temperature.  These methods typically use
either the daily maximum and minimum temperatures, or the daily average
temperatures.

Personally I'm of the opinion that the the details of the approximation
method used are not important, so long as it uses the data it's given to
generate degree-day figures that are very close to those that would be
generated by the *correct* method (or, more realistically, by a method that
used half-hourly temperature readings or similar)."
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20111120/4c9d601b/attachment.html>